Images posted on social media show an engine with the exterior cover missing as the plane approaches Honolulu. Video shows the engine shaking back and forth with pieces of the cover flapping in the wind.

"There was a loud bang ... and then the plane really started shaking," passenger Allison Sudiacal told Hawaii News Now . "There was a loud boom and then it was like rattling and the plane was kind of shaking like boom, boom, boom."

Sudiacal was traveling with her 4-month-old son, her husband, Tim, and his parents.

"They let us know that we had to brace for impact in case there was a rough landing. It was scary. But they did a really good job," Sudiacal said.

An emergency was declared due to a vibration in the right engine, said Federal Aviation Administration spokesman Ian Gregor said in an email, adding that the agency will investigate the incident.

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Associated Press writer Paul Elias in San Francisco contributed to this report.

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